Electroetching tool using localized application of channelized flow of electrolyte

ABSTRACT

An electroetching tool using scanned localized application of flowing electrolyte against a workpiece such as a large area mask having high density features for the fabrication of microelectronic components. A masked molybdenum plate is suspended in a vertical direction within a tank which functions as a reservoir for a recirculating electrobyte. The electrolyte in the reservoir is filtered and pumped to a pair of travelling cathode assemblies from which the flowing electrolyte is simultaneously applied through respective charged orifices to both sides of the workpiece. The workpiece is masked on its opposite sides with mirror imaged mask apertures having corresponding opposite-sided features in registration with each other. 
     Each orifice through which the electrolyte is applied comprises an open groove in the surface of a block of polyvinal chloride material which groove extends in a vertical direction relative to the tank. The bottom of the groove is adjacent to a conductive plate. The open top of the groove is held closely against the masked plate as the cathode assembly is moved along the guide rails. The fresh electrobyte is introduced to the groove at the upper end thereof while the used electrolyte exits from the groove at the lower end thereof and into the tank reservoir for recirculation.

BACKGROUND OF THE INVENTION

The present invention generally relates to electroetching and, moreparticularly, to the electroetching of relatively large area maskshaving high density features for the fabrication of microelectroniccomponents.

Chemical etching is well known in the art for making metal maskssuitable for use in the fabrication of microelectronic components.Chemical etching frequently employ highly corrosive and hazardousreagents. Such processes also produce toxic wastes requiring veryexpensive treatment to meet the increasingly stringent environmentallaws. Furthermore, the chemical etching process is slow and has limitedresolution. Electroetching techniques reduce the severity of suchconcerns but have not been used for making metal masks.

Electroetching additionally offers better control over metal removalthan does chemical etching. Compared to chemical etching which generallyis isotropic, a certain degree of anisotropy can be achieved withelectroetching. Due to the shadow effect of photoresist used to definethe areas to be etched, there is some restriction to the direct accessby the current in these locations whereby etch rates are locally reducedand a degree of anisotropic etching effect is produced. Other advantagesof electroetching include high metal removal rate and the possibility ofetching varieties of metals and alloys, including corrosion resistantmetals, in electrolytes with minimized safety and waste disposalproblems.

U.S. Pat. No. 3,962,056 issued on Jun. 8, 1976 to Shakir A. Abbas,discloses a modified electroetching process for forming holes withvertical sides in a masked monocrystalline silicon wafer. Impurities areintroduced through registered mask openings on both sides of the siliconwafer. The wafer then is anodically etched through mask openings on oneside of the wafer to form porous silicon regions completely through thewafer at the locations of the openings. The resultant porous siliconregions are removed with a porous silicon etchant simultaneously appliedthrough the mask openings to both sides of the wafer.

U.S. Pat. No. 3,730,861 issued on May 1, 1973 to W. A. Haggerty teachesanother electroetching technique intended to produce precisely shapedworkpieces. The technique positions an electro-chemical machining toolat a preset distance from a rotating anodic workpiece while directing astream of cathodic electrolyte against the workpiece.

Neither of the exemplary cited patents addresses the fabrication ofmetal masks by electroetching and the problems thereof of maintaininguniformity of current and electrolyte distribution over the workpiece,especially where the workpiece is a relatively large area mask havinghigh density features for the fabrication of microelectronic components.

SUMMARY OF THE INVENTION

One purpose of the present invention is to provide an electroetchingtool for making large area workpieces, characterized by high densityfeatures, while maintaining current and electrolyte distributionuniformity over the workpiece.

Another purpose is to provide an electroetching tool specially adaptedfor the fabrication of large area, high feature density masks for makingmicroelectronic components.

A further purpose is to provide an electroetching tool forsimultaneously etching opposite surfaces of a large area workpiece,characterized by high density features, while maintaining current andelectrolyte distribution uniformity over the workpiece.

These and other purposes of the present invention, as will appear from areading of the following specification, are achieved in a best modeembodiment by the provision of an electrolyte tank in which a maskedmolybdenum plate is suspended in a vertical direction from a fixedlocation. The tank functions as a recirculating electrolyte reservoir.The electrolyte in the reservoir is filtered and pumped to a travellingcathode assembly from which the flowing electrolyte is selectivelyapplied through a charged orifice to at least one side of the workpiece.The workpiece is maintained at a positive electric voltage relative tothe travelling cathode. In the best mode embodiment, the molybdenumplate is masked on its opposite sides with mirror imaged mask appertureshaving corresponding features in registration with each other. A pair ofregistered cathode assemblies are caused to travel. between guide railsstraddeling the tank so that flowing electrolyte is appliedsimultaneously to registered opposite locations on the opposite-sidedmasked plate.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified perspective view of the structural elements ofthe best mode embodiment of the present invention;

FIG. 2 is a cross-sectional view of one of the travelling cathodeassemblies of FIG. 1;

FIG. 3 is a block diagram representing the electrolyte circulationapparatus and the electrical potential sources associated with theembodiment of FIG. 1; and

FIG. 4 is a cross-sectional view of one etched hole in a doubly-maskedworkpiece produced by the embodiment of FIG. 1, as a function of theapplied electric voltage.

BEST MODE FOR CARRYING OUT THE INVENTION

The electroetching tool of the present invention utilizes a technique ofthe localized dissolution of a masked workpiece by flowing electrolytefrom the charged orifice of a moving cathode which is scanned across theface of the workpiece. Referring to FIG. 1, a workpiece or mask holder 1is fixed to the sidewalls of electrolyte reservoir tank 2. A maskedmolybdenum plate 3 is inserted in holder 1. Both sides of plate 3 arecovered by a resist mask for delineating features where holes of desiredshape and size are desired to be placed in plate 3. The two resist masksare mirror images of each other with corresponding features inregistered relationship on opposite sides of plate 3.

Two movable cathode assemblies 4 and 5 are mounted on guide rails 6 and7 via transverse support member 8 which drivable as shown by the doubleheaded arrow 9 so as to scan assemblies 4 and 5 in synchronous unisonacross the respectively opposing surfaces of workpiece 3. Each cathodeassembly comprises a narrow charged orifice extending in a verticaldirection relative to tank 2 so that flowing electrolyte is applied inthe presence of an electric field simultaneously to registered oppositelocations on the double sided masked plate 3. The assembly is shown inmore detail in FIG. 2.

As shown in FIG. 2, the orifice is in the form of a groove 10 cut intothe face of polyvinyl chloride block 11. Embedded adjacent a surface ofblock 11 is an electrically conducting plate 12 to which a voltagepotential is applied via electrodes 4 and/or 5. Flowing electrolyte 14enters the upper end of groove 10 from ducts 15 and exits from the lowerend of groove 10 into the reservoir tank 2 of FIG. 1. Electrolyte 14 issubstantially confined to flow within groove 10 and against the face ofworkpiece 3 of FIG. 1 by the close presence of workpiece 3 across whosemasked face the open side of groove 10 is placed flush and is closelyscanned. A precision linear motion unit (not shown) is used to move thecathodes over the workpiece at a preset speed.

FIG. 3 is a block diagram of the entire apparatus of the presentinvention incorporating the components of FIGS. 1 and 2. FIG. 3 furtherrepresents the electrolyte recirculation system and the manner in whichelectric potentials are applied to both of the cathode assembliesrelative to the anodic workpiece. The electrolyte 14 is stored inreservoir 2, is pumped (16) through filter 17 and is distributed to thetwo cathode assemblies via valve 18, flow monitor 19 and via valve 20,flow monitor 21, respectively. The workpiece 3 is connected to thepositive poles of power supplies 22 and 23. The negative poles ofsupplies 22 and 23 are connected to cathodes 5 and 4, respectively. Twoseparate power supplies 22 and 23 are employed for better control of thecharges applied between cathodes 5 and 4 and the workpiece 3. Powersupplies 22 and 23 preferably include integrators and have thecapability to preset the amount of charge expended during etching.

Cathode plate 12 is fixed on block 11 at a distance from the entrance ofelectrolyte 14 into groove 10. This distance, known as the entrancelength, in principle should be approximately 50 times the hydraulicdiameter of the channel in order to provide fully developed velocityprofiles at the active surface of the electrodes. Some baffle provision(not shown) can be placed in the electrolyte entrance to help compensatefor the effect.

The cathode groove dimensions and the electrolyte flow are importantparameters obtaining uniform current distribution across the workpiece.The cathode length should be slightly larger than the active length ofthe workpiece. The electrolyte flow is determined by the hydraulicdiameter of the groove 10 which for a given length is established by thegroove width and the spacing between the workpiece 3 and the cathodes 4and 5. In order to localize the dissolution of the workpiece and toobtain high current density, a very narrow, groove 10 is used, e.g., ofthe order of about 0.25 inches to about 0.50 inches. A very smallspacing of about 2-3 mm between assemblies 4 and 5 and workpiece 3provides the attainment of high flow rate and improved directionality offlow.

FIG. 4 demonstrates the relationship of the sidewall shape of thethrough-holes resulting from two-sided electroetching as a function ofthe electric voltage applied between cathodes 4 and 5 and workpiece 3.Such differing sidewall shapes may be required, depending on the natureof the features to be etched into the mask 3 for example by utilizingphotoresist 27. For example, molybdenum masks for screening purposesrequire that the walls of the holes be straight as at 24 of FIG. 4.Knife-edged vias as represented by the numeral 25 would be more suitableif the finished mask 3 were to be used as a C4 (ball joint) evaporationmask. The differing sidewall shapes shown in FIG. 4 are achieved bycontrolling the amount of etching, as by controlling the amount ofapplied electric voltage. Increasing the potential incrementallyproduces the shape sequence denoted by 25, 24, 26. It should be noted,however, that the fabrication of features having differing pattern sizesand densities require that the current distribution be extremely uniformover the entire workpiece in order to achieve shape uniformity. Suchuniform current distribution is provided by the electroetching tool ofthe present invention by the technique of using the small scanningcharged orifice of the described cathode(s) and integration of itseffect across the entire face of the workpiece.

What is claimed is:
 1. An electroetching tool comprising a tank forholding electrolyte,means for holding a masked workpiece plate to beetched, said workpiece plate adapted to be positioned in a verticaldirection when said workpiece is in said tank, at least one cathodeassembly having a narrow rectangular orifice extending In said verticaldirection within said tank, means for causing said electrolyte to flowfrom said tank and through said orifice for impingement upon said platewhen in place, means for scanning said assembly so that said orifice ismoved in a direction perpendicular to said vertical direction across theface of said plate, and means for applying an electric potential acrosssaid plate and said assembly, said assembly being negatively energizedrelative to said plate, whereby uniformity of current distributionacross the face of said plate is achieved by the integrated effect ofscanning the orifice of each said at least one cathode assembly acrossthe face of said workpiece.
 2. The electroetching tool defined in claim1 wherein said orifice comprises a vertically extending groove on thesurface of said assembly when facing said plate,said assembly beingadapted to closely scan across the facing surface of said plate when inplace, said electrolyte being caused to flow from the upper end of saidgroove to the lower end thereof.
 3. The electroetching tool defined inclaim 2 and further including an electrically conductive plate adjacentthe bottom of said groove,said electric potential being applied to saidconductive plate so as to make said conductive plate negative relativeto said workpiece plate when in place.
 4. The electroetching tooldefined in claim 3 wherein said conductive plate is fixed adjacent thesurface of a block of insulating material.
 5. The electroetching tool ofclaim 2 wherein the spacing between said groove and said facing surfaceof said plate is from about 2 mm to about 3 mm.
 6. The electroetchingtool of claim 2, wherein the width of said groove is from about 0.25inches to about 0.50 inches.
 7. The electroetching tool defined in claim1 and including a pair of cathode assemblies, said pair of assembliesbeing scanned in unison across opposite faces of said plate when inplace,means for masking said plate in mirror image fashion wherebycorresponding features are in registered relationship on opposite sidesof said plate when in place.
 8. An electroetching apparatus for etchinga masked workpiece, said apparatus comprising:a tank for holdingelectrolyte, holder means for fixedly positioning said masked workpiecein a vertical direction within said tank, at least one cathode assemblyhaving a narrow rectangular orifice extending in said vertical directionwithin said tank, means for causing said electrolyte to flow from saidtank and through said orifice to impinge upon said masked workpiece,means for scanning said assembly so that said orifice is moved in adirection perpendicular to said vertical direction across the face ofsaid masked workpiece, and means for applying an electric voltage acrosssaid holder and said assembly, said assembly being negatively energizedrelative to said holder, and thereby, forming said electroetchingapparatus for etching a masked workpiece.
 9. The electroetchingapparatus defined in claim 8, wherein said orifice comprises avertically extending groove on the surface of said assembly facing saidmasked workpiece,said assembly being closely scanned across the facingsurface of said masked workpiece, and wherein said electrolyte is causedto flow from the upper end of said groove to the lower end thereof. 10.The electroetching apparatus defined in claim 9, and further includingan electrically conductive plate adjacent the bottom of said groove,saidelectric potential being applied to said conductive plate so as to makesaid conductive plate negative relative to said masked workpiece. 11.The electroetching apparatus defined in claim 10, wherein saidconductive plate is fixed adjacent the surface of a block of aninsulating material.
 12. The electroetching apparatus of claim 9,wherein the spacing between said groove and said facing surface of saidmasked workpiece is from about 2 mm to about 3 mm.
 13. Theelectroetching apparatus of claim 9, wherein the width of said groove isfrom about 0.25 inches to about 0.50 inches.
 14. The electroetchingapparatus defined in claim 8, and including a pair of cathodeassemblies, said pair of assemblies being scanned in unison acrossopposite faces of said masked workpiece,said masked workpiece beingmasked in mirror image fashion whereby corresponding features are inregistered relationship on opposite sides of said masked workpiece.